This section provides background information related to the present disclosure which is not necessarily prior art.
In a motor vehicle having an all-wheel drive that may be switched in, at least a part of a driving torque of a drive unit is permanently transmitted to a primary axle of the motor vehicle. A part of the driving torque may also optionally be transmitted to a secondary axle of the vehicle by means of a friction clutch (e.g., a multi-disk clutch).
A desired clutch torque is calculated for this purpose in an associated control unit on the basis of vehicle state parameters—such as a wheel speed difference between the primary axle and the secondary axle, a steering angle, and a yaw rate of the vehicle. To set this desired clutch torque at the all-wheel drive clutch of the motor vehicle, a control signal is fixed for a clutch actuator in accordance with a predetermined desired clutch torque/control signal relationship, with this control signal corresponding to a specific adjustment path of the actuator and thus of the clutch. The desired clutch torque/control signal relationship may, for example, be stored as a function or as a table. This relationship typically has a substantially linear development in the relevant range so that this relationship may be characterized by an offset value and a gradient. The actuation of the all-wheel drive clutch by means of the associated actuator may take place, for example, electrically, electromechanically, electromagnetically, hydraulically, or magnetorheologically.
The desired clutch torque/control signal relationship for the control of the clutch actuator may be fixed, for example, by an ex works calibration of the clutch. However, this relationship can change over the course of time due to wear phenomena so that the clutch torque set by means of the actuator no longer corresponds to the desired clutch torque value. To avoid this effect, a readjustment of the actuator based on a setting precision observation is required. A direct measurement of the torque transmitted by means of the all-wheel drive clutch is, however, undesirably complex and/or expensive.
It is known from DE 103 46 671 A1 to calculate the actual clutch torque from the drive slip at the front axle, from the drive slip at the rear axle, and from the total driving torque. However, this method requires that the drive slip at the front axle and the drive slip at the rear axle are known as separate values or that the constants for the calculation of the driving torque transmitted at the front axle and at the rear axle may be cut, which does not always correspond to the conditions actually prevailing in practice.
DE 103 46 673 A1 describes a method in which an adjustment path is preset for the clutch actuator and would have to result in a fully blocked state of the clutch in accordance with a characteristic; and a check is made whether a slip occurs for the transmission clutch. For this purpose, the clutch must, however, be brought into the blocked state, which can undesirably influence the handling of the motor vehicle, in particular when this state should be maintained over a longer period to increase the precision of the method by averaging the measured values.